About the Deck Calculator
This deck calculator estimates the main materials for a rectangular deck surface: the number of decking boards, the joists that support them, the deck screws or hidden fasteners, and the material cost per square foot. Enter the deck length (the direction the boards run) and width, pick the board width, stock length and joist spacing, and it lays out the rows of boards the way a builder would.
It is built for DIYers pricing a pressure-treated or composite deck, homeowners comparing a contractor’s material list, and anyone replacing old decking on an existing frame. Board count uses total linear footage with a waste allowance, which reflects how offcuts are reused when joints are staggered across joists.
The estimate covers the decking and joists only. Beams, posts, footings, ledger board, stairs and railings depend on height, span tables and local code — confirm joist size and spacing against your decking manufacturer’s instructions (composite boards often require 12-inch spacing, and 16 inches is typical for wood).
With the default inputs, the decking boards is 29. Change any value above to recalculate instantly.
How to use the deck calculator
- 1Enter the deck length in the direction the boards will run, and the width.
- 2Choose the board width, board length you can buy, and the gap.
- 3Pick joist spacing — 16 in for wood, often 12 in for composite.
- 4Set a waste allowance and enter board and joist prices.
- 5Review boards, joists, screws and cost per square foot.
Formula and method
Boards run along the deck length and are laid in rows across the width. Each row takes one board width plus one gap, so the number of rows is the width in inches divided by (board width + gap), rounded up. Rows × deck length gives total linear feet of decking; adding the waste allowance and dividing by the stock length gives the number of boards to buy, assuming offcuts are reused when staggering joints.
Joists run across the width, spaced along the length at the on-center spacing, with one extra joist to close the end: ⌈L × 12 ÷ s⌉ + 1. Face-screwing uses two screws wherever a board row crosses a joist, so screws = rows × joists × 2. Cost is boards × board price plus joists × joist price.
- L, W
- Deck length (board direction) and width (ft)
- b
- Actual board width (in)
- g
- Gap between boards (in)
- s
- Joist spacing on center (in)
Worked examples
16 × 12 ft wood deck with 16 ft boards
144 in of width ÷ (5.5 + 0.1875) in per row = 25.3, so 26 rows of 16 ft boards (416 linear ft). With 10% waste that is 29 boards. Joists at 16 in on center across 192 in need 12 spaces + 1 = 13 joists, and the materials cost $872, about $4.54 per sq ft.
20 × 14 ft composite deck, joists at 12 in
A 14 ft width holds 30 rows of 5½ in boards, giving 600 linear ft; with 10% waste that is 33 twenty-foot composite boards. Twelve-inch spacing along 20 ft needs 21 joists, and the decking plus joists cost $2,073.
10 × 10 ft platform with 2×4 decking
Narrow 3½ in boards with a 1/8 in gap need 34 rows across 10 ft, or 340 linear ft. With waste that is 32 twelve-foot boards; joists at 24 in need 6 pieces, for $372 in material.
Frequently asked questions
How many deck boards do I need for a 12 × 16 deck?+
With 5½ inch boards and a 3/16 inch gap, a deck 12 ft wide needs 26 rows. If the boards run the 16 ft direction that is 416 linear ft, or about 29 sixteen-foot boards with 10% waste.
What joist spacing should I use for decking?+
Wood decking is usually installed over joists at 16 inches on center. Many composite and PVC boards require 12 inches for diagonal layouts or thinner profiles; always follow the manufacturer’s span rating.
How much gap should be between deck boards?+
Common gaps are 1/8 to 1/4 inch. Kiln-dried and composite boards are often gapped at 3/16 inch, while wet pressure-treated lumber is sometimes butted tight because it shrinks as it dries.
How many screws do I need per deck board?+
Face-screwing uses two screws at every joist the board crosses. A 16 ft board over joists at 16 inches on center crosses 13 joists, needing 26 screws; hidden fastener systems use one clip per crossing.
Does this include beams, posts and railings?+
No. It covers decking boards and joists. Beam size, post spacing, footings, ledger attachment, stairs and railings depend on deck height, loads and local building code, so size them from span tables or a designer.